Synchronous reluctance motors having rotor segments of extended pole span

ABSTRACT

In reluctance motors having rotors with segmentally disposed, axially extended, radially laminated stacks of steel strips carried on a conducting core, improved synchronous performance and increased asynchronous torque are achieved when the rotors are modified to associate only one segmental body with each pair of adjacent MMF poles of the stator field. The modification is actually a simplification, namely the span of each segmental body is peripherally extended to nearly the span of two adjacent stator poles, whereby the steel laminae carry a higher peak magnetic flux centered on the strips lying at the radial middepth position of a stack. A higher total flux is also realized and the flux distribution in the air gap is more nearly sinusoidal, lacking the gaps characterizing flux patterns of prior art rotors. Odd-numbered harmonic coefficients of the flux distribution are thus minimized. Considerable savings in construction are also realized. Motors are described ranging from one pair of rotor poles up to four pairs, although the invention extends to any practicable higher pole numbers.

United States Patent Menzies [451 June 20, 1972 SYNCHRONOUS RELUCTANCE2,989,655 6/1961 Honsinger ..3 10/162 x MOTORS HAVING ROTOR SEGMENTS OFEXTENDED POLE SPAN Primary Examiner-J. V. Truhe Assistant ExaminerB. A.Reynolds [72] inventor: Robert W. Menzies, Winnipeg, Manitoba, Atorney-Roman J, Filipkowski Canada [57] ABSTRACT [73] Assignee: CanadianPatents and Development i i Ottawa, Ontario, Canada In reluctance motorshaving rotors with segmentally disposed, axially extended, radiallylaminated stacks of steel strips car [22] Flled: 1971 ried on aconducting core, improved synchronous per- [21] APPL No: 107,640formance and increased asynchronous torque are achieved when the rotorsare modified to associate only one segmental body with each pair ofadjacent MMF poles of the stator field. [52] {1.8. CI ..3l0/163,310/183, 310/197, The modification is actually a simplification, namelythe span 310/212 of each segmental body is peripherally extended tonearly the [51] Int. Cl. ..H02k 19/06 span of two adjacent stator poles,whereby the steel laminae [58] Field of Search ..3 10/162, 163, 183,197,211, carry a higher p magnetic flux centered on the strips y g310/212, 166 at the radial mid-depth position of a stack. A higher totalflux is also realized and the flux distribution in the air gap is more[56 1 References Cited nearly sinusoidal, lacking the gapscharacterizing flux patterns of prior art rotors. Odd-numbered harmoniccoefficients of UNITED STATES PATENTS the flux distribution are thusminimized. Considerable savings I in construction are also realized.Motors are described ranging 2,939,025 5/1960 W1ll1ford ..3 l0/l63 Xfrom one pair of rotor poles up to four pairs although the 1355394612/1927 Clause 3 10/162 UX vention extends to any practicable higherpole numbers. 2,733,362 1/1956 Bauer et al..... ....3l0/l62 v 3,448,3106/1969 Lawrenson ..3l0/l 62 8 Claims, 9 Drawing Figures PATENTEDmzo m23,671,789

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saw u or 4 galm m SYNCIIRONOUS RELUCTANCE MOTORS HAVING ROTOR SEGMENTS FEXTENDED POLE SPAN This invention relates to synchronous electricmachines of the reluctance type wherein the rotor is assembled fromstacks of nested curved strip elements, each element extending parallelto the rotor axis.

The improvement with which the invention is primarily concerned is theprovision in a synchronous electric machine of a rotor constructionhaving segmental magnetic bodies made of steel laminae stacked in theradial direction wherein the circumferential span of the segmental bodyis increased to span two consecutive stator MMF poles of oppositepolarity, whereby higher power factor, higher efficiency, and greatersynchronous torque are made possible, with the added advantage that thecost of rotor construction is markedly lowered.

The present invention is related to, and improves on, the synchronouselectric machines proposed in a paper authored by Cruikshank, Menziesand Anderson in PROC. IEE Vol. 113, No. 12, December 1966, wherein thefabrication of a rotor is described, involving the winding ofresin-coated grainoriented strip steel about a mandrel to form a unitarythick ring, cutting the ring into a number of like segmental bodies, andassembling the bodies symmetrically about the rotor shaft with the sideedges of the strips machined to form cylindric surface portions of therotor. In such prior art rotors, adjacent machined surfaces ofconsecutive segmental bodies share a common polarity, requiring that therotor have as many segmental bodies as the number of stator MMF poles,or more properly, since no rotor direct axis passes through anysegmental body, twice as many segmental bodies are required as thenumber of stator pole-pairs. The costs of fabrication of sixpole andeight-pole rotors are high, being larger for higher pole numbers forwhich a number of wound rings may be required.

A magnetic defect inherent in the sharing of stator pole flux byadjacent segmental bodies reduces the density of directaxis rotor fluxover the surface of each rotor core lobe below the optimum value,reducing maximum synchronous torque and decreasing the power factor ofsuch prior art motors.

According to the present invention, a segmental rotor is fabricated bycutting the wound steel ring into segments of such angular span thateach segmental body spans an arc of the stator manifesting a pair ofadjacent MMF poles of opposite polarity. Each rotor pole thus comprisesone cylindrically machined surface formed by the margins of a singlestack of laminations, and hence presents a continuous arcuate surface ofuniformly low reluctance to the stator MMF. Not only are obviouseconomies in fabrication confirmed, but far superior synchronouspull-out torque, power factor, and improved asynchronous performanceresult from the improved sinusoidalflux distribution in the air-gap andthe higher fiux density achieved.

The invention will be more particularly described in the followingdisclosure in conjunction with the accompanying figures of the drawingwherein:

FIG. 1 is a cross section of a reluctance motor having a rotor of theprior art form carrying six segmental bodies, operating in a statorhaving windings to provide three MMF pole-pairs, the rotor direct axesbeing shown registered on the stator oles; p FIG. 2 is a cross sectionof the same motor provided with a rotor comprising but three segmentalbodies according to the present invention, operating in thethree-pole-pair stator field of FIG. I; with the rotor direct axes shownalso registered on the stator poles;

FIGS. 3 and 4 respectively are developed representations showinggraphically the air gap flux densities for the motors of FIGS. 1 and 2;

FIG. 5 is a cross section of an alternative motor having afour-segmental body rotor according to the invention operating in afour-pole-pair stator MMF field;

FIG. 6 shows an alternative rotor construction providing FIG. 7 shows across section of another reluctance machine having a single segmentalrotor body lacking any rotor core, for use in a stator wound to providea single pole-pair MMF field;

FIG. 8 is a perspective view of the rotor of FIG. 7 showing clamping andshaft support structure; and,

FIG. 9 is a cross section of a rotor having a bar core carrying twosegmental bodies for operating in a stator providing a twopole-pair MMFfield.

Before proceeding to the description of the preferred embodiments itwill be helpful to review the prior knowledge of induction motors andsynchronous reluctance machines, for a fuller comprehension of the novelprinciples and constructions contributed by this invention.

OPERATING PRINCIPLES OF ELECTRIC POLYPHASE RELUCTANCE MOTORS Thesynchronous reluctance machine has a stator comprised of planar annularsteel sheet elements, laminated axially in a manner to minimize eddycurrent losses, windings being carried in slots recessed into theinternal surface and being energized so that at any instant themagnetizing force manifests a magnetic field having an even number ofpoles of alternate polarity uniformly spaced about the internalcircumference. The MMF at any instant has a sinusoidal arcuatedistribution. Such form of stator provides a rotating field just as doesthe stator of a polyphase or single phase induction motor or a polyphaseor single phase synchronous machine.

In contrast with the rotor of a conventional induction motor which ismade as an axially extended stack of discs carrying a conductiveshort-circuited cage winding in slots adjacent the rotor surface, thestack being characterized by uniform permeability in any radialdirection, the rotor of the reluctance machine is characterized bymagnetic asymmetry, i.e. the magnetic material is arranged to presentwidely differing permeability over specific surface portions. Certainsurface zones extending parallel with the shaft exhibit low reluctanceto magnetization by stator MMF and will be permeated by a large fiuxwhen a stator pole is presented to such zones, while other zonesintermediate the high permeability zones exhibit very large reluctance.The magnetic asymmetry is designed to produce a sinusoidal fluxdistribution in the air gap when the poles of the rotating stator MMFare registered on the low reluctance zones of the rotor. At zero load,the rotor locks on the stator field and rotates synchronously with it inthis relative position. With increased load, the relative position ofthe rotor and the rotating field changes by a small angle, developing ahigher torque as the angle increases. At a certain critical load angle,i.e. when the angle between the rotor direct axis and the stator fieldpole axis exceeds about 45 (on the basis that an angle of 360 issubtended between a point on a stator pole of one polarity and acorresponding point on the next-following stator pole of the samepolarity) the rotor pulls out of step and slows down, running as aninduction motor.

In the publication referred to, a large increase in air-gap flux densityin the direct axis of rotor magnetization has been shown to berealizeable by employing segmental bodies made up of nested curvedgrain-oriented strip steel, the easy direction of magnetization of whichis along the lines of intersection with the strips of a segmental bodyby a radial plane normal to the shaft axis. In such prior rotor elementthe segmental bodies were so dimensioned as to span from about 1 15 toof the stator field pole pitch, i.e. the pair of zones of low reluctanceon one segmental body are presented toward a pair of stator MMF poles ofopposite polarity. Each fraction of the span of one MMF pole of thestator field. At zero load in synchronous running, the bar is oppositestator areas over which the field is zero or negligibly small.

COMPROMISES BETWEEN ASYNCHRONOUS AND SYNCHRONOUS DESIGN OPTIMIZATION FORRELUCTANCE MACHINE ROTORS In the prior publication, it was pointed outthat an optimum pole span of the rotor segmental bodies for synchronousrunning to attain maximum pull-out torque would be as close to 100percent as structural integrity may tolerate, since optimum torque andpower factor are related to maximum flux density in the air-gap in therotor direct axes and to minimum flux density in the air-gap in therotor quadrature axes. The maximum torque T is proportional to:

(XD J i XDXQ and the maximum power factor, Cos is proportional to D Ql(xv x0) where:

X,, is stator reactance for the rotor position registering thedirect-axes on the MMF poles of the stator field; and

X is stator reactance for the rotor position registering thequadrature-axes on the MMF poles of the stator field.

Because it will usually be necessary to accelerate the rotor by thedevelopment of asychronous torque, in the machine rather than byapplying external drive, it is necessary to design the rotor foreffective operation at sub-synchronous speeds as a cage-wound rotor foran induction motor.

The magnitude of the sub-synchronous torque T depends on the conductingmaterial which forms low-resistance closed circuits about the rotorwherein large currents may circulate due to voltages induced by rotorslip relative to stator flux. Since the entire volume of the rotor corein which the segmental bodies are carried may be a homogenous and highlyelectrically conducting mass of metal such as aluminum, and at least onemassive bar of the same material can be disposed in the rotor surfacecentrally of each segmental body and welded-to the core, the very lowresistance loops thus provided achieve excellent accelerating torque anddamping.

Referring now to FIGS. 1 through 6, the improvement in increaseddirect-axis flux density realized in rotors constructed according to theinvention will be clearly made evident from the comparison of theair-gap flux density and its distribution in the prior art narrowpole-span segmental body form with the flux density and distributionachieved in the widened polespan segmental body form of the presentinvention.

PRIOR ART ROTOR CHARACTERISTICS A reluctance machine according to theprior art is shown in diametral section in FIG. 1, and in FIG. 3 theair-gap flux density is graphically represented in Cartesiancoordinates.

For purposes of illustration the machine may be considered to besupplied from a three-phase electrical supply (not shown) led to thefield winding terminals T T,, T,. A known form of stacked-disc statorbody 11 carries in slots 12 recessed into its interior surface adistributed three-phase winding, only one coil 13 of which isillustrated. The arrangement of conductors wound as coil groups instator slots is believed to be well understood by those aware of the artof induction motors. Such windings, arranged by known techniques,produce at any instant a sinusoidal MMF distribution in the air-gap 14between the stator 11 and the rotor 15, and in the present example suchfield will be assumed to comprise 6 poles, i.e. three pole-pairs.

Assuming that the motor is operating as a synchronous motor with zeroload, the effect of current in the phase windings will be to set up thefield pattern graphically depicted for a specific instant of time by thedashed envelope line 16 traced externally about the periphery of thestator, the ordinate value and the direction of the field being depictedby the radial arrows extending from the reference circle 1. Suchpattern, of course, would be revolving at an angular speed, in

radians per second, of:

where:

dO/dt is angular velocity in radians per second,

f is supply frequency in Hertz,

P is the total number of stator poles.

Rotor 15 essentially consists of a massive central core 17 revolubleabout an axis 2 centered in the support shaft 3. The core is formed of aconductive non-magnetic metal such as aluminum or its alloys. It isshaped as an axially extended body having six lobes 18 extending intothe rotor surface and terminating in narrow arcuate sectoral surfaces 19located equidistant from stator 1 1. Twelve zones of the rotor cylindricsurface exhibiting low reluctance occupy the sectors designated F andare arranged in pairs on either side of lobes 18, thus manifesting sixrotor poles, while other surface portions occupy sectors F exhibitinghigh reluctance, i.e. these zones are difficult to magnetize.

The core 17 is formed with six trough-shaped seating surfaces 20disposed between the lobes, in which the segmental bodies 2126 arerespectively secured. Each segmental body comprises a radially stackedgroup of closely nested thin curved strips or sheets such as thatindicated by numeral 27, the sheet material being a grain-oriented steelhaving high unidirectional permeability. The direction of easymagnetization is aligned with any plane passed through the rotorperpendicular to the axis 2 so that the reluctance of a path extendingfrom one sector F bounding the exposed side edges of the strips of onestack to a corresponding sector F bounding the other side edges of thesestrips is extremely low, whereas the reluctance of a path in the axialdirection is much higher,

while the reluctance of a path intersecting the sheets at right anglesis greatly higher still.

Each stack of steel sheets is held in radially compressed condition by aretaining bar extending parallel with the rotor axis 2 and having theouter side formed as a cylindrical surface lying in the rotor surface.Such retaining bars are preferably also made of a non-magnetic metalsuch as aluminum or copper, and are preferably supported on the core bythreaded bolts 28 of non-magnetic high-strength alloy such as astainless steel passing through bores 29 piercing each steel sheet 27midway between the side edges and being threadedly engaged by theirinner ends in suitably threaded bores 30 in shaft 3. Conic heads 31 areseated in countersunk recesses 32 in the outer face of a bar 100.

Considering now the MMF curve 16 representing the locked on relationshipof MMF field and rotor structure, a field pole of one polarity, forexample an N" pole has its peak intensity adjacent a core lobe surface19 and hence develops rotor magnetization of the same polarity in thepair of adjacent surfaces F for instance in segmental bodies 26 and 21.The next consecutive field pole, proceeding clockwise around axis 2, isan 5" pole similarly straddling that core lobe which spaces apart thesurfaces F of segmental bodies 21 and 22, developing rotor magnetizationin these surfaces which is of a polarity opposite to that in the pair ofsurfaces of bodies 26 21. The rotor therefore has as many poles as thetotal MMF poles comprising the stator field, or twice as many segmentalbodies are required as the number of pole-pairs of MMF.

The MMF field adjacent the sectors F which are centered on the retainingbars 100 is negligibly small or zero. The quadrature axes Q-Q maytherefore be drawn as three diametral lines bisecting pairs of retainingbars 100 and also bisecting the associated segmental bodies 21, 24, 22,25, and 23, 26. The rotor direct axes Z-Z similarly comprise a set ofthree diametral lines bisecting the opposed core lobes 18 and alsobisecting the peak intensity MMF field plots.

The flux distribution will therefore be traced along six closed-looppaths designated by solid lines 33 indicating the mean path followed bythe flux in the stator and along a curved portion within the segmentalbodies, the sense of the flux being shown by directional arrow markings.

Referring now to FIG. 3, the interrupted envelope line 34 representsordinate values of flux density above and below the horizontal referenceline xx representing the rotor periphery and showing therealong therelative positions of the direct axes Z-Z and quadrature axes -0. Itwill be evident that although portions of the line 34 more or lessconform to the desired sinusoidal flux distribution, virtuallynegligible flux is set up along a narrow band 35 centered on each directaxis position. This may be understood from FIG. 1 wherein the core lobes18 will be seen to present surfaces of very high reluctance centeredopposite peak MMF plots. It may also be seen that if the completesine-curve had been traced, a higher peak flux density and a largertotal flux would be represented in place of the gaps 35.

The high reluctance of the rotor sectors F is shown by insignificantfiux density in the air gap over a relatively wide band 36 centered oneach quadrature axis position Q-Q, hence such rotors have low X asdesired. However the maximum X value is less than ideal, so thatcorrespondingly reduced maximum torque T and power factor Cos (by areevident from relations (1) and (2). Not only is the fundamental Fouriercomponent of the flux-density distribution diminished, but higherodd-numbered harmonic components, particularly the fifth, have largeamplitudes, causing harmonic time-varying torques and induced harmonicvoltages at subsynchronous speeds that are detrimental to the startingof the motor. The magnitudes of the undesirable harmonic components offlux density distribution will be apparent from the following relation:B, I" p g sin (a p B,,)2b,, sin n(p0mtp8,)..(3) where phase a isconsidered the leading phase with current i I-sin (mt a) and f is themaximum amplitude of the fundamental component of stator MMF;

g is the effective radial length of air-gap (14);

u is the permeability of free space;

B, is the angular position of the rotor quadrature axis 0-0 with respectto the axis Z--Z of stator phase a;(t=0) and,

b, is the Fourier coefficient of the n spatial harmonic fluxdensitydistribution.

When the rotor direct axes Z-Z are aligned, as illustrated, with thepeak intensity positions of the stator MMF poles, then p B a 1r, 2

and equation (3) may be written:

um n-I Bd sin n (p0tpB,,)

where and is the magnitude of the n' harrnonic component of the air-gapflux density distribution.

For n l the fundamental direct-axis flux density distribution and hencethe direct-axis reactance X which is proportional to B should clearly beas large as possible. As 8,, is directly proportional to b,,, the valueof b should be maximum, while the harmonic coefficients (i.e. n =3, 5, 7should be as small as possible in order to prevent the development ofharmonic torques and induced harmonic voltages. Within mechanical designlimits, chiefly dictated by considerations of securing the segmentalbodies to the rotor core and of providing adequate cross-sectional areasof cage circuits carrying damping currents, the arcuate spans of lobes18 and of bars 100 may be varied. An inspection of the waveform 34 willshow that these parameters control the width of gaps 35. Those skilledin motor design art will be aware that a given variation of thesearcuate spans will minimize predetermined odd-numbered harmonics, butcannot augment the fundamental. In general, in carrying out a specificmachine design, an accurate analysis and computation of air-gap fluxdensity distribution will be carried out to arrive at optimum values ofb,.

As an example of prior art reluctance machine analyses, the b Fouriercoefficient for the flux density distribution curve 34 in the gap 14between a stator of four MMF pole-pairs and a rotor carrying eightsegmental bodies, was calculated to have an amplitude 30 percent of thefundamental.

PREFERRED RELUCTANCE MOTOR EMBODIMENTS Referring now to FIGS. 2 and 4, areluctance motor 10 according to the invention has a stator 11 andwindings such as 13 identical in all respects with the stator andwindings of the machine of FIG. 1, but has a modified rotor comprising acore 117 with three lobes l8 and three concave seating surfaces 120, onwhich are mounted three segmental bodies 121, 123, and 125. Theconstruction of these segmental bodies is in accordance with thedescription of similar bodies in the prior art machine of FIG. 1, i.e.they comprise stacks of nested curved laminations extending in the axialdirection, having strip margins exposed in the rotor cylindrical surfaceoccupying the arcuate spans F which have a sectoral span which is thelarger part of one stator MMF pole span. The direct axes Z-Z arerepresented by three diametral lines, which will each be seen to bisecta pair of opposite sectors F Each segmental body such as 121 spans amaximum arcuate length nearly equal to the span of two consecutivestator MMF poles. Three diametral lines Q-Q representing rotorquadrature axes altemately pass through the rotor core 117 bisecting thecylindrical surfaces 19 of lobes l8, and bisecting the sectors F i.e.passing through the radially deepest part of a lamination stack.

An examination of the air-gap flux density distribution represented bythe waveform 134 in FIG. 4 shows that the distribution is more nearlysinusoidal than the corresponding envelope 34 of the prior art machineof FIG. 3. Moreover, the maximum flux density is about 20 percent higherthan the maximum amplitude in waveform 34. It will be immediatelyapparent that the total flux in the air gap in the machine of FIG. 2 isconsiderably higher than in the prior art machine because there is nogap in the peak-flux density position.

Table I shows the magnitudes of Fourier coefficients of the air-gap fluxdensity distribution for reluctance machines having stator fields ofsix-pole and eight-pole form, rotors A being of prior art form whereineach rotor pole comprises one surface sector F, of each of a pair ofadjacent segmental bodies, and rotors B being constructed according tothe present invention wherein each rotor pole comprises the surface F ofa single segmental body spanning nearly one stator MMF pole.

TABLE I Reluctance machines SIX-pole Eight-pole "ZX- TJ ITXH .H T

When the stator MMF field in the machine of FIG. 2 has rotated out ofsynchronous locked on relation so as to align the rotor quadrature axeswith the peak MMF positions, presently illustrated as coinciding withthe ZZ axes, it will be evident that virtually negligible flux would beset up either along the laminations, or transverse to their thickness.This will be evident from a consideration that each strip will have itsexposed margins disposed in field positions of the same polarity andintensity. As has previously been stated, the reluctance along a rotorQ-Q axis is low due to the interlaminar films of bonding resin spacingsuch strips apart radially. Thus, the modification of the segmental bodyform of rotor according to the invention does not result in any increasein the value of Xq, and the efficiency will be high.

The greater magnetic cross section of rotor segmental body materialavailable for conducting a high total magnetic flux and realizing highair-gap flux density in multipole reluctance machines constructedaccording to the invention will be particularly apparent from a study ofthe motor embodiments illustrated in FIGS. 2, 5, 6, 7, 8 and 9.Considering FIG. 5, wherein a rotor 215 comprises four segmental bodies221,

' 222, 223 and 224, operating in a stator 11 providing four the samerotor diameter and rotor pole number it will be found that the volume ofeach of the four segmental bodies is somewhat larger than the combinedvolumes of the pair of adjacent segmental bodies which would carry theflux of one stator MMF pole in the prior design consisting of eightsegmental bodies. The pole flux, represented by the closed path 233,will also be found to be considerably greater. If the arcuate width ofthe cylindrical surfaces 19 of lobes 18 is narrowed still further andthe concave seating surfaces 220 deepened in the core 217 accordingly,the stack of laminations nested therein may be correspondinglythickened, gaining still greater pullout torque.

Where the motor is required to develop large asynchronous torque whilerunning up as an induction motor, and yet have good pull-out torqueunder synchronous operation, the form of rotor 315 of FIG. 6 may beutilized. In this construction, interlaminar films or layers 37 aredeliberately thickened and interleaved with the steel. Each segmentalbody is thus divided into a number of subbodies, for example three,namely, 121a, 1211:, 121a; 122a, 122b, 122c; and 123a, 123b, 123e,wherein the curved steel strips are wound on mandrels of differentradii. As diagrammed, segmental bodies 125ca have respective radii r,,r,, r, swung on centers P P P lying radially outward from the rotorsurface along a QQ axis at progressively decreasing distances, so thatthe nearer the sub-body lies to the rotational axis 2 of the rotor, thelonger is its generating radius.

When the interlaminar material 37 occupying the spaces between adjacentsegmental sub-bodies is a conductive metal integrally bonded with enddisc structure (as shown in FIG. 8) which structure is itself integrallybonded to the core 117, the cross-sectional area of the damper loops soprovided is high. The retaining bolts 28 may be of electricallyconductive material but should be non-magnetic to preserve low X, Theshaft 3 may be of magnetic material such as steel as in otherembodiments having through-shafts.

It will be apparent that rotors of machines according to the presentinvention may be constructed to operate in stators providing fields ofany feasible number of MMF pole-pairs, only one segmental body beingrequired for each pole-pair. The lowest possible number of segmentalbodies is one; in FIG. 7 a cross section is shown of a motor having arotor 415 formed as a single segmental body 321, operating in a statorproviding an MMF field of one pole-pair, as depicted by the dashedoutline or waveform 16 relative to the circular reference line 1. InFIG. 8 the rotor and shaft construction is shown in perspective view. Itwill be evident that the laminae 27 are all flat strips, extending inchordal planes parallel with the axis 2, clamped by a pair of retainingbars 100 engaged by transverse non-magnetic bolts 28 passing through theentire rotor assembly and being threadedly engaged in sleeve 40.

While two flux paths 333 are shown as though extending in oppositehalves of the stator, the magnetic flux set up in the segmental body 321is actually unidirectional, i.e. the rotor is truly a 2-pole device, andthe flux is distributed over each sector F in accordance with the MMFpotential between opposite margins of each strip. The rotor direct axisZ-Z comprises one diametral line parallel with the laminae, and isintersected at right angles by the single diametral line -0 representingthe rotor quadrature axis.

The clamping bars 100 have their ends electrically and mechanicallybonded, as by welding, to non-magnetic electrically conductive end discs41, while the stack of steel strips is held compressed. It is the bars100 together with the integral end discs which form the electricallyclosed circuit in a plane transverse to the laminae which provide alarge damping capability.

Stub shafts 3A secured rigidly in bores 42 coaxial with the end discs 41protrude axially of the rotor. The end discs should be amply thick toprovide the required rigidity and strength.

Machines of the form of FIGS. 7 and 8 obviously comprise the most simpleconstruction of the invention, and have the highest synchronous speed.

The embodiment shown in H6. 9 is similar to that of FIGS. 5, 7 and 8except that the rotor 515 comprises two magnetically separate segmentalbodies 421, 422, and operates in a stator providing an MMF field of twopole-pairs. As explained for FIG. 7, end discs 41 are integrally bondedwith the central massive electrically conductive non-magnetic supportingbar 317 of aluminum or its alloys, the bar having parallel side faces18A, 18B disposed equidistantly from the axis 2 of shaft 3 centered inthe bar. Cylindrical surfaces 19 lie radially equidistant from the axis2 in the rotor cylindric surface at gap 14.

Each segmental body 421, 422 comprises a radially stacked group ofclosely nested flat sheets or strips such as sheet 27 having theirdirection of easy magnetization aligned in the chordal direction of therotor, there being two sectors F, representing individual rotor polesper segmental body. Each body is held in compressed condition by theretaining bar which subtends an angle of about one radian at the shaftaxis 2, the bars being engaged by fastening means such as bolts orrivets 28 having their inner ends secured in the shaft 3 as by threadedconnection 29, 30 illustrated.

When such form of rotor is disposed in a stator with the two diametrallines Z-Z aligned with the peak intensity positions of four MMF fieldpoles of the stator, the magnetic flux loops linking both stator androtor will be as represented by the mean paths 433.

Zero or negligible flux will be set up across the segmental bodies inthe 0-0 axis, due to the very high reluctance of the bar core 317.

From the foregoing, it will be apparent that regardless of the number ofpole-pairs provided by the stator winding, motors according to thepresent invention will have the highest possible pull-out torque,surpassing the pull-out torque for any prior art reluctance machine,while objectionable harmonics induced in the circuits will be minimized.

lclaim:

1. An electric motor of the reluctance type having a wound stator memberand a rotor member revolvable in said stator and spaced therefrom by anannular air-gap, said stator having a winding providing, when energized,a rotating MMF field comprising a predetermined number of pole-pairs,said rotor comprising a non-magnetic damping structure, and a number offerro-magnetic bodies of segmental form carried by said dampingstructure adjacent said air gap, said number of bodies corresponding tosaid predetermined number of stator polepairs, each of said segmentalbodies having a pair of direct magnetization axes intersecting said bodyand spanning an arc corresponding to the angular span of two consecutiveMMF field poles.

2. An electric motor as claimed in claim 1, wherein said dampingstructure comprises conducting end discs, and integral conducting barelements each associated with a corresponding segmental body,respectively, for joining said end discs and disposed adjacent saidair-gap, each segmental body and the single bar element associatedtherewith being bisected by a quadrature axis.

3. An electric motor as claimed in claim 2 wherein said predetenninednumber is one.

4. An electric motor as claimed in claim 2 wherein said predeterminednumber is two or more.

5. An electric motor as claimed in claim 4 wherein said dampingstructure further comprises non-magnetic core structure extendingaxially between said end discs and being integral therewith and havinglobe portions extending radially and bounded by said air gap, andwherein a quadrature axis bisects each lobe.

6. An electric motor as claimed in claim 5 wherein said core has twoopposed lobes and supports two segmental bodies, each body comprising agroup of radially stacked axially elongate flat sheets having highpermeability along at least one dimension of the sheet.

7. An electric motor as claimed in claim 4 wherein said core has threeor more equiangularly disposed lobes and has concave seating surfacesdisposed between the lobes, each seating sheets of respective sub-stacksdecreasing inversely with distance of the sub-stack from the core, andsaid damping structure includes conductive non-magnetic materialinterleaved between said sub-stacks and bonded electrically with saidend discs.

1. An electric motor of the reluctance type having a wound stator member and a rotor member revolvable in said stator and spaced therefrom by an annular air-gap, said stator having a winding providing, when energized, a rotating MMF field comprising a predetermined number of pole-pairs, said rotor comprising a non-magnetic damping structure, and a number of ferro-magnetic bodies of segmental form carried by said damping structure adjacent said air gap, said number of bodies corresponding to said predetermined number of stator pole-pairs, each of said segmental bodies having a pair of direct magnetization axes intersecting said body and spanning an arc corresponding to the angular span of two consecutive MMF field poles.
 2. An electric motor as claimed in claim 1, wherein said damping structure comprises conducting end discs, and integral conducting bar elements each associated with a corresponding segmental body, respectively, for joining said end discs and disposed adjacent said air-gap, each segmental body and the single bar element associated therewith being bisected by a quadrature axis.
 3. An electric motor as claimed in claim 2 wherein said predetermined number is one.
 4. An electric motor as claimed in claim 2 wherein said predetermined number is two or more.
 5. An electric motor as claimed in claim 4 wherein said damping structure further comprises non-magnetic core structure extending axially between said end discs and being integral therewith and having lobe portions extending radially and bounded by said air gap, and wherein a quadrature axis bisects each lobe.
 6. An electric motor as claimed in claim 5 wherein said core has two opposed lobes and supports two segmental bodies, each body comprising a group of radially stacked axially elongate flat sheets having high permeability along at least one dimension of the sheet.
 7. An electric motor as claimed in claim 4 wherein said core has three or more equiangularly disposed lobes and has concave seating surfaces disposed between the lobes, each seating surface supporting one segmental body therein, and each body comprising a group of radially stacked axially elongate nested curved sheets having high permeability along at least one dimension of the sheet.
 8. An electric motor as claimed in claim 7 wherein each said segmental body comprises a plurality of sub-stacks of curved sheets in radially stacked relation, the radii of curvature of the sheets of respective sub-stacks decreasing inversely with distance of the sub-stack from the core, and said damping structure includes conductive non-magnetic material interleaved between said sub-stacks and bonded electrically with said end discs. 